Method of brazing



2 Claims. tel. ze -4st This applicationis a division of application Serial No. 724,486, filed March 27, 1958, now US. Patent No.

3,065,537, issued November 27, 1962. 1

Our invention relates to an improved brazing composition which issuitable for use in a high temperature and corrosive environment, and to a method for preplacing said brazing composition.

Molten metals and molten salts find considerable use in industry as coolants due to their high heat capacity,

high heat transfer coefficients, and low vapor pressures at elevated temperatures. Increasing interest in the use of those materials is being shown in nuclear reactor development. For example, sodium and sodium-potassium alloys are being used in reactor heat-exchange circuits. However, these media are corrosive and present difficult problems in the fabrication of reactor components.

.In the fabrication of components for heat-exchange I systems, items such as flanges, piping, and tanks may be easily fabricated using welding techniques. However, complex and delicate components such as fin-to-tube heatexchangers are not amenable to fabrication by welding. Efforts have been directed, therefore, towards fabrication of the more complex reactor components using brazing methods, and stringent requirements must be met. For

example, many commercial brazing compositions and fluxes are eliminated from consideration because of high thermal neutron cross-sections. Others are severely attacked by corrosive media and nuclear radiations. An: other serious difficulty is in the preplacement of the brazing alloy on the surfaces to be joined. Electrolytic preplacing ofbrazing materials is not generally satisfactory because of inability to deposit a uniform plate on a complex surface.

'One object of our invention, therefore, is to provide an improved method of joining metal surfaces for use in hightemperature, corrosive media.

Another object isto provide an improved brazing composition and method of preplacing said brazing composition in the joining of metal surfaces, wherein the resulting joint is resistant to oxidative and corrosive attack.

Another object is to provide a brazing composition that displays adequate mechanical strength and heat transfer characteristics and does not have a high total neutron absorption cross-section. i

Still another object is to provide a method of performing said brazing in a relatively simple and rapid manner.

United States Atomic Energy United States Patent O with the reaction:

Other objects and advantages of our invention will bei come apparent from the following detailed description and the claims appended hereto.

In accordance with our present invention, metal surfaces may be joined by preplacing a brazing composition comprised of a major portion of nickel, a minor portion of phosphorus and a minor portion of chromium on said surfaces, and brazing together the resulting surfaces. The resulting alloy displays superior oxidation and corrosion resistance at elevated temperatures under prolonged exposure to molten metals; the metallurgical bond possesses adequate mechanical strength and heat transfer characteristics; and the alloy is satisfactory from the thermal neutron cross-section view-point. The brazing composition may be evenly preplaced on the surfaces to be joined,

. the nickel-phosphorus layer.

, areas-s7 Patented Jan. 3.2, 1 965 and. during brazing displays excellent flow, wetability, and filleting properties. 7

.T he composition of the brazing allow may vary over a considerable range, although the major. constituent must be nickel. We generally find that the nickel content of the alloy should be at least approximately 70% by weight, a particularly suitably alloy comprising, for example, approximately 5 %-15 phosphorus, by weight, and approximately 5%l5%, by weight, of chromium, theremai-nder being nickel. Our preferred brazing composition comprises approximately: nickel, 10% phosphorus and 10% chromium. This composition has a flow pointin the.

region of l750l800 F.

Thebrazing composition may be preplaced upon the metal surfaces to be joined in a variety of ways, and the exact method employed is not critical. One suitable method involves thechemical deposition of nickel and phosphorus and the subsequent electroplating. of chromium on the deposited nickel and phosphorus.

In this method, nickeland phosphorusare first deposited from an aqueous bath by reaction betweennickel ions and ananionic complex of phosphorus and oxygen,

such as phosphite and hypophosphite; Inonesueh reaction, employing nickel chloride and sodium hypophosphite, the hypophosphite is converted to the phosphite with subsequent deposition of nickeLperhaps in accordance NiCl +NaH PO +H O Ni+2HCl-l-NaH PO Since the deposited nickel is found to contain phosphorus, secondary reactions of unknown mechanism probably play a significant role in the deposition. The bath should be operated at close to a neutral pH and at-an elevated temperature, say'about 9.0- C. The rate of deposition isslow, being only about 0.00035 inch per hour, and to get a plate of the desired thickness, approximately 0.001 inch, may take several'hours. Upon completion of the nickel-phosphorus deposition, the resulting platedsurfaces may be made the cathode of a conventional electrolytic cell, and a very thin layer. of chromium electroplated thereon. For the preferred composition the chromium layer should be approximately one-tenth the thickness of Once the Ni-P-Cr mixture has been applied to the surin a variety of conventional ways, and the exact method employed is not critical to the success of our invention. One satisfactoryv method comprises placing the nickelphosphorus-chromium covered surfaces in a dry, nonoxidizing atmosphere, preferably a hydrogen atmosphere, for about 10 minutes at a temperature above the melting point of the brazing composition, and this produces an alloy of the brazing constituents. For the preferred composition of 80 percent nickel, 10 percent phosphorus,.l0 percent chromium the preferred brazing temperature is approximately 1800 F. As the nickel-phosphoruschromium mixture melts, it seeks the nearest capillary, and adequately fills unplated metal along a joint to form a uniform fillet along the full length of the joint.

Our brazing method may be satisfactorily used in joining a wide variety of metals with a melting-point higher than the melting point of the brazing composition. All nickel and iron-base alloys may be joined using our method. Typical of the commercially available alloys which may be joined by our method are all the stainless steels and the alloys sold as the Nichromes, Inconels and Chromels.

The following example will illustrate our invention in greater detail.

Example Fins for a fin-to-tube heat exchanger were prepared by punching 0.190 inch holes in 0.010 inch type 304 stainless .every thirty minutes.

steel sheet. A lip was left to provide a fin per inch spacing. These fins were plated with a nickel-phosphorus composition, using the following procedure. The fins were degreased, anodically etched in 37% HCl for one minute at'roorn temperature using 6 volts, in order to clean the surface. The resulting fins were given a nickel strike for five minutes at 2 volts, at a temperature of 7080 F. and at pH 0.5 in a bath containing one gallon of water, 40 ounces of NiCl -6H O and 4 ounces of boric acid. The fins were rinsed, washed in 37% HCl and immediately placed in nickel-hypophosphite bath made as follows: 27 1 grams sodium carbonate were dissolved in distilled water and the solution was boiled with the slow addition of glycolic acid to give pH 7. The solution was cooled, and then filtered. Distilled water was added to the filtrate to make about /3 liter and 30 grams of nickel chloride were dissolved therein. 10 cc. hypophosphite solution, prepared by dissolving 100 grams sodium hypophosphite in 100 cc. distilled water, were added, the resulting solution was filtered and enough distilled water was added to the filtrate to make 1 liter. The bath was then maintained at 90-95 C. and pH 6, using NaOH or HCl to maintain the pH. About two cc hypophosphite solution was added The rate of deposition was approximately 0.00035 inch per hour, and the deposition was continued until a thickness of about 0.020 of an inch was obtained. Onto this plate approximately 0.002 inch of chromium was plated by conventional electrodeposition means. The resulting fins were then fitted onto 6 inch O.D tubes having a wall thickness of 0.020 inch. This assembly was brazed at 1800 F. for 10 minutes in a dry hydrogen atmosphere, and excellent flow, wettability and filleting were observed. A portion of the specimen was then evaluated for corrosion resistance in molten sodium at 1500 F. The extent of the attack after 100 hours exposure time was negligible; and the fillet remained relatively sound with only minor attack at the fillet-sodium interface. Another portion of the assembly was exposed to static air at 1500" F. for 500 hours and virtually no oxidation was revealed.

The above example is merely illustrative and should not be construed as limiting the scope of our invention. It should be apparent that those skilled in metallurgy may make variations in our method without departing from the spirit of our invention. Therefore, our invention should be limited only as indicated by the appended claims.

Having thus described our invention, we claim:

1. An improved method of joining structural metal surfaces of metals selected from the group consisting of nickel-base and iron-base alloys which comprises placing said surfaces in an aqueous bath, contacting in said bath nickel ions with an anionic complex of phosphorus and oxygen, thereby depositing nickel and phosphorus on said surfaces, electroplating chromium on said nickel and phosphorus covered surfaces, the conditions for depositing nickel, phosphorus and for electroplating chromium being controlled to produce a composite layer of about percent nickel, 10 percent phosphorus, and 10 percent chromium, and heating the resulting plated surfaces to a temperature of about 1800 C. in a dry non-oxidizing atmosphere.

2. The method of claim 1 wherein said anionic complex is introduced into said bath as dibasic sodium phosphite and said nickel ions are introduced into said bath as nickel chloride.

References Cited by the Examiner UNITED STATES PATENTS 1,939,467 12/33 Short et al 29498 X 2,398,449 4/46 Ronci 29497 X 2,464,821 3/49 Ludwick et al 29497 X 2,714,760 8/55 Boam et a1 29498 X 2,795,040 6/57 Antel et al 29502 X 2,961,759 11/60 Weissfioch 29502 X 2,962,811 12/60 Herbert 29-502 X 2,987,817 6/61 Kozlik 29502 X 3,073,269 1/ 63 Hoppin et al 29504 X JOHN F. CAMPBELL, Primary Examiner. 

1. AN IMPROVED METHOD OF JOINING STRUCTURAL METAL SURFACES OF METALS SELECTED FROM THE GROUP CONSISTING OF NICKEL-BASE AND IRON-BASE ALLOYS WHICH COMPRISES PLACING SAID SURFACES IN AN AQUEOUS BATH, CONTACTING IN SAID BATH, NICKEL IONS WITH AN ANIONIC COMPLEX OF PHOSPHORUS AND OXYGEN, THEREBY DEPOSITING NICKEL AND PHOSPHORUS ON SAID SURFACES, ELECTROPLATING CHROMIUM ON SAID NICKEL AND PHOSPHORUS COVERED SURFACES, THE CONDITIONS FOR DEPOSITING NICKEL, PHOSPHORUS AND FOR ELECTROPLATING CHROMIUM BEING CONTROLLED TO PRODUCE A COMPOSITE LAYER OF ABOUT 80 PERCENT NICKEL, 10 PERCENT PHOSPHORUS, AND 10 PERCENT CHROMIUM, AND HEATING THE RESULTING PLATED SURFACES TO A TEMPERATURE OF ABOUT 1800*C. IN A DRY NON-OXIDING ATMOSPHERE. 